Photoelectric controlled carrier modulator



Patented Mar. 22, 1949 UNITED ES- FA'IENTA PHOTOEIJECTR'IC" C'ONTRODIEDCARRIER MODULATOR Hughv C. Ressler, Bayside,

Wireless,.-Inc., NewYork, NnY., a. corporation of Delaware N. Y.,assigner to Press In certainkinds of signalling systems, such as'Afacsimile transmission, it is' desirable totransmit thefacsimile'signals inthe'form of a modulated tone which, for example, may be ofvaudio frequency. It is also desirable tobeabletocontrol the modulationsdirectly by means `ofthe photo electric cell which responds to theshadecharacteristics of the subject matter to be transmitted. Variouskinds of' these directcell-.modulators have .been proposed- Infgeneral,such prior. modulators are based upon the use of the cellina` balancedWheatstone bridge network, whereinthe varying impedances of the cell areconverted into bridge unbalanced currents.. The tone signal or carrieris applied to the bridge across one set of conjugate points, andthe-modu lated tone-isv taken offy froml the remaining two coniugatepoints.

However, with these prior arrangements', there are a-number ofdisadvantages which it isthe purpose of the present inventionftoovercome. Since the tone signal is in the nature ofan. alternatingcurrent. of. audio frequency, itis necessary toemploy specially designedand expensive coupling transformers of the balanced` winding type.Secondly, in order to balance. out the residualimpedance of thephoto-electriccell, it.. is necessary to'employ'a separatesimilar cell.:or to provide a specially'designed duplex` photoelectric cell, and caremust bel taken to make sure that the capacity of the various leads.- toVtheduplex cell are fully balanced.. Thirdly, .with the prior Wheatstonebridge type of. tone modulatorit is not easy to-balance outthe inherent.cell impedance over av wide range of incident light values. For example,if with the. prior arrangement the carrier. is balanced out for onegiven value of incident light, it is` not. easy to4 balance out thecarrier for other widely different. light values.

Accordingly, it is a principal object. of this. invention to provide animproved tonemodulator controlled. by a photo-electric cell wherein. theamplitude of the tone appearing at the output.. of the modulator can bevariedaccordingtothe level of incident light and balancedV out for any,desired datum or reference levelof incident light normally encountered.v

Another object is to provide a photo-electric cell carrier.modulatingnetwork of the parallel-1'. resistance-capacitance type,wherein. the tone source can be .directly connected=to the network.

2 without. requiring special input coupling transformersand the like.

A` feature of the invention relates' toia modu-l lating network of theparallel-T type for use' in producing aimodulated tone or carriersignal,

and wherein the input tone signal or carrier can;

bereadily balanced out.

Another feature relates to an improved' carrier.. modulation controlnetwork employing a `photo-electric cell, wherein the input tone signalor carrien can be suppressed by a relativelysimple: combmatlon ofresistors and. condensers, thusv enabling. the standard two-terminalphoto-elem tric. cell: to be used for a wide variety ofworkingrequirements.

Another. feature relates to a photo-electric cellv carrier modulatoremploying a "parallel-T networkwhereby part of the network together withthe..cellcanbeassembled as a separate unit for n lounti'ng4 on. thefacsimile machine chassis, and

the remainder of the network can be located ata-.remote .pointconvenient to the tone source andconnectable to the facsimile machineyby a'y shielded. cable or the like, which forms a com- I mon.returnfor. the network.

Other features and advantages not particu-- larly enumerated: will beapparent after a` consideration of the following detailed descriptions:

and.the1appended. claims.v In the drawing whichA tion,

Fig. lis a simplified schematic diagram of a. modulatmg networkembodying features of the invention.

35.y Fig..2` isaschematic diagram of a facsimile transmittersystemembodying the network simila-rto-that of Fig. 1.

Fig. 3 is a schematic diagram of a modication-ofFig. 1..

iication` of Fig. 1.

Thephotocell modulator herein described cani bey generally classified asa three terminal network` containing two kinds ofr impedance ele- Y.mentsonly, namely resistances and capacities.

The generator or tone source is connected to'v one of the terminals andto the common terminalfreferred to as ground; the loador utilizationcircuit is connected to a second terminal andl tothe. common or groundterminal.

-rent from the generator is transmitted to the shows. one preferredembodiment of the inven-A Fig..4 is .a schematicdiagram of anothermodi--Y pedance elements in one of the current branches so that thecurrents arriving at the load over the two separate paths are equal inmagnitude but opposite in phase at the carrier frequency, when the cellis illuminated at the reference level. The circuit can be designed tobring about a condition of carrier suppression at a second referencelevel of illumination by a further adjustment of the impedance elementusually called the balance control. This type of modulator in contrastto the conventional Wheatstone bridge, balances at a single frequencyonly but can be made to balance at any desired reference level of cellillumination normally encountered.

The network can take the form of a parallel- T network shown in Figs. 1and 2 in which the two current paths are through the T sections, or itmay take a more general form, a ladder network with a bridging elementshown in Figs. 3

and 4, wherein the current paths are through the ladder itself andthrough the bridging element.

Referring to Fig. 1, the numeral I represents any suitable source ofalternating current, for example a tone or carrier current generator ofaudio frequency, and numeral 2 represents any suitable utilizationcircuit to be worked with modulated carrier. The units I and 2 areconnected together through a network 3 according to the invention. Thisnetwork may be referred to generally as a parallel-T network. One T-section comprises the condenser 4, and photoelectric cell 5 in series,and with their `junction point P connected through resistor 6 to thecommon return conductor I0. The secondT-sec tion comprises the tworesistors 1 and 8 in series, and with their junction point P1 connectedthrough variable condenser 9 to the common return I. The return II) maybe a common ground return or it may be a separate conductor such forexample as the sheath of a sheathed cable.

From the foregoing, it will be seen that there are no D. C. potentialsapplied to the cell 5, and the maximum swing of the carrier ispreferably such as not to subject the cell to any A. C. voltages above 2volts. With such an arrangement the cell operates substantially byreason of its apparent changes in impedance in response to incidentlight signals. However, even in the absence of light, there is always asmall inherent impedance between the electrodes, the effect of whichmust be balanced in order that zero carrier may exist at the terminalsII, I2, when the cell is completely dark.

I have found that by suitable adjustment and selection of the values forresistors 6, 1 and 8, and condensers 4 and 9, it is possible directly tomodulate the carrier from source I, merely by con--y trolling the lighton the cell 5. Furthermore, the f balancing of the inherent impedance ofthe cell 5 and the suppression of the carrier at terminals I I and I2can be effected for no light incident on the cell. Thus, in certaincases, it may be required to have zero tone or carrier corresponding toa black shade of a picture. In that event, the network is initiallyadjusted by means of condenser 9 to the desired balance with the cell 5completely screened, resulting in substantially en-`v tire balancing-outof the carrier applied to the utilization circuit 2. Therefore, whenlight signals of increasing intensity from black to white are impressedon cell 5, the level of the carrier is correspondingly increased. Inother cases, it may be desirable to work with a suppressed carrieramplitude corresponding for example to a picture shade of gray. In thatevent, the network must be initially balanced by having a mean lightshade e. g., gray incident on cell 5. Therefore, values of light aboveand below this mean light shade value will result in correspondingvariations in the amplitude modulation of the carrier from source I. Inorder to effect this result, the twin T network of Fig. 1 is modified asshown in Fig. 2. The modification consists in adding a. section composedof a condenser in series with, and a resistance in shunt to, the Tsection containing the cell. Thus, as shown in Fig. 2, the twin Tsection comprising the elements 4, 5, 6, together with the modifyingelements is in parallel with the T section comprising the elements l, 8and 9. An additional section composed of .adjustable resistor I8 andcondenser I 9, is connected in cascade with the T section 4, 5, 6, andthis combination is connected in parallel with the T section 'I, 8 and9. Suppression of the carrier at any given value of illumination is thenaccomplished as follows. Variable resistor I8 is set to its maximumvalue. The cell 5 is screened from light and the carrier is balanced outby adjusting condenser 9. When the correct setting of condenser 9 isfound, it is left in its adjusted position. For suppression of thecarrier at anyother desired reference level of illumination, resistor I8is reduced in value until the carrier is again reduced to minimum orzero with this reference level of illumination on the cell.

Heretofore, where Wheatstone bridge modulating networks have beenemployed for this purpose, it has not been easy to balance the bridge tozero setting to correspond to widely different values of light on thecell. In such cases, it has been necessary either to use different cellsor a critical adjustment of the potentials applied to the bridge isnecessary. Furthermore, in these prior systems great care must be takenthat the input transformer connecting the carrier source to the bridgeis properly balanced and the lead wires connecting the network to thephoto cell must likewise be critically balanced. These priorarrangements therefore, have necessitated either the use of two separatephoto cells, one corresponding to cell 5 which is the main or signalresponsive cell, and the other an auxiliary cell for balancing-out thecapacity of the main cell. In other cases, it has been necessary toemploy duplex photo-electric cells having three separate lead-in wires,the balance between which must be accurately adjusted. With thearrangement described in connection with Fig. 1, the cell 5 is capableof operation with complete elimination of vthe inherent impedance of thecell, and zero tone signal can be produced corresponding to any desireddatum level of light incident on the cell 5.

` One of the important advantages of the modulated network abovedescribed is the fact that the cell 5 and the balancing condenser 9 canbe assembled as a unit for convenient mounting anywhere on the frameworkof .a facsimile machine. Thus, as shown in Fig. 2, the cell 5 togetherwith the resistors 1, 8, and the balancing condenser 9, are assembledinside the usual photoelectric cell box or housing I3, which can bemounted so that the cell 5 receives the light reflected from the subjectmatter on the scanning drum I4 in the usual way. The input tothe box I3can be supplied by means of a sheathed twowire cable, Athe 4conductorsIof which are designated I5, I6, and -the sheath of which is desig-ynated I'I. Elements I8, I9, 4 and yIi shown in Fig. 2 can be placed onthe stationary part of the facsimile machine, remote from the travellingphotocell box. The circuit elements shown can be so lproportioned that'the capacity to ground introduced by the cable, as well as couplingcapacity between leads I5 and I6 either have a negligible effect on thecircuit performance, or their effects can easily be accommodated bymeans of the balancing elements. Element I8, the carriersuppressioncontrol, is usually placed on the control panel of the unit. Carriertone is supplied to the modulator through the cable leads I5 and IB andthrough the comm-on return and cable sheath Il. A concentric cable isIprovided to connect the output of the modulator to the utilizationcircuit. This consists of the concentric Vconductor II and the sheathI2.

As one typical example of an arrangement found to produce the desiredresults, the various components of Fig. 2 had the following values.

Ra= megohms, R1=3 megohms, Rc=80,000 ohms, R1s=110,000 ohms Variable,C9=180 mmfd. Variable, 04:.002 mfd., C19=.03 mid., frequency of source 11800 C. P. S. Ph-otocell 5 is a type RCA934.

Fig. 3 shows a -modic-ation of Fig. 1, wherein instead `of employing.parallel-T networks, a ladder network is employed .composed of seriesresistors 25, 26, 2l, 28, with shunt condensers 29, 30, 3I, the latterone of which is adjustable and corresponds in function to the balancingcondenser 9 of Figs. 1 and 2. The photo-electric cell 5 is bridgedacross the entire network. The remaining parts of Fig. 3 whichcorrespond functionally to those of Fig. 1, bear the same designationnumerals. By lproper choice of the elements constituting the network andby appropriate adjustment of the condenser 3 I, for a given tone e. g.,1800 C. P. S. applied to the input terminals and with no light on cell5, the -currents arriving at the output terminal II Iover the twoparallel paths represented respectively by the network land the cell 5,will be equal in magnirtude but opposite in phase. If it should bedesired to balance out the carrier at the terminal II for any referencelevel lof light other than black, a similar arrangement is employed suchas shown in Fig. 4. This arrangement is substantially the same .as thatof Fig. 3 except that an additional section comprising condenser 32 andvariable resistor 33 is connected vacross the input end of the network.When it is desired to balance out the tone or carrier at the terminal II corresponding for example to a gray light on cell '5, the condenser 34is adjusted with the cell 5 completely screened against light so as toproduce substantially zero or minimum carrier at terminal II. Duringthis adjustment, the resistor 33 has been set at its maximum Value. Cell5 is then exposed to the gray reference light, and resistor 33 islowered in value until the carrier at the terminal I I is again reducedto minimum or zero.

Fig. 4 shows one particular set of values for the various resistors Iandcondensers that were found to achieve the desired result at an inputtone frequency of 1800 C. P. S. It will be understood of course, that ifthe tone signal is a different frequency from 1800 C. P. S., the variouselements will lbe accordingly designed to produce 'the desiredbalancing-out of the carrier at the load circuit.

While certain embodiments of the invention have been disclosed herein,it will be understood that various changes and modifications may be madewithout departing from the spirit and scope of the invention.

What is claimed is:

1. A modulator arrangement for telegraph systems and the like,comprising a carrier source, a carrier modulating network comprising twoparallel paths, one path including in series a photo-electric cell and acapacitance, the other path including in series a pair of resistances, acommon return point from said two paths to said source, a resistorconnecting the first of said two paths to said common path and acondenser connecting the other of said pair of paths to said commonpoint, said resistor and condenser serving to suppress the carrier forany given value of light on the cell.

2. A modulator arrangement for telegraph systems and the like,comprising a carrier source, a utilization circuit to be energized by asignalmodulated carrier, a line connecting said source and said circuit,said line including a phase-shifting network, one arm of which comprisesin series a photo-electric cell and a capacitor, the other arm of whichcomprises in series a pair of resistors, means connecting said line toone pair of points of said network, and a resistor and condenserconnected to another pair of points of said network; and means to adjustsaid network to suppress the carrier to correspond with anypredetermined light level on said cell.

3. A modulator arrangement for telegraph systems and the like,comprising a carrier source, a. utilization circuit to be excited bysignal-modulator carrier, a line connecting said source and circuit,said lne connecting a phase-shifting network, one arm of which includesin series a photoelectric cell and a capacitance, the other arm of whichincludes in series a pair of resistors, means to adjust said network tobalance-out the carrier, means connecting said line to one pair ofpoints of said network, and a resistor and condenser connected toanother pair of points of said network, the last-mentioned condenserserving to suppress the carrier for any desired light condition on saidcell.

4. In a modulating system of the character described, a source ofcarrier current, a utilization circuit to be worked with modulatedcarrier, means to modulate said carrier in accordance with lightvariations and comprising a connection between said source and circuit,which connection includes two parallel paths, one of said paths beingsubstantially entirely capacitative and including a light-sensitive celland a series capacitor, the other of said paths being substantiallyentirely resistive, a common return for said source and circuit, andadjustable impedance means between said paths and said common return forsuppressing the carrier at the input of said circuit when said cell issubjected to a predetermined light intensity.

HUGH C. RESSLER.

REFERENCES CITED The following references are of record in the iile ofthis patent:

UNITED STATES PATENTS Number Name Date 1,992,055 Cockrell Feb. 19, 19352,054,836 Shepard Sept. 22, 1936 2,177,366 Iams Oct. 24, 1939 2,191,795Kolb Feb. 27, 1940I 2,298,466 Cooley Oct. 13, 1942 2,407,293 ShepherdSept. 10, 1946

